Early  Geological  History 
of  Chicago 

BY 

HENRY  W.  NICHOLS 
Associate  Curator  of  Geology 


Geology 

Leaflet  7 


FIELD  MUSEUM  OF  NATURAL  HISTORY 

CHICAGO 

1925 


LIST  OF  GEOLOGICAL  LEAFLETS  ISSUED  TO  DATE 

Model  of  an  Arizona  Gold  Mine $  .10 

Models  of  Blast  Furnaces  for  Smelting  Iron    .        .10 
Amber — Its  Physical  Properties  and 

Geological  Occurrence .10 

Meteorites 10 

Soils 10 

The  Moon 10 

Early  Geological  History  of  Chicago        ...        .25 

D.  C.  DAVIES,  Director 


FIELD  MUSEUM  OF  NATURAL  HISTORY 
CHICAGO.  U.  S.  A. 


No. 

1. 

No. 

2. 

No. 

8. 

No. 

4. 

No. 

5. 

No. 

6. 

No. 

7. 

Field  Museum  of  Natural  History 

DEPARTMENT  OF  GEOLOGY 
Chicago,  1926 

Leaflet  Number  7 

Early  Geological  History 
of  Chicago 

This  geological  history  of  Chicago  may  properly 
begin  with  that  period,  the  Cambrian,  when  for  the 
first  time  evidences  of  life  were  abundantly  recorded 
in  the  rocks.  This  time  is  so  inconceivably  remote  that 
Prof.  Barrell  has  estimated  that  six  hundred  million 
years  have  since  elapsed.  We  know  something  of 
world  conditions  in  even  more  ancient  times,  but  the 
rocks  of  that  more  ancient  era  are,  in  the  vicinity  of 
Chicago,  deeply  buried  and  at  best  the  records  they 
hold  are  obscure  and  difficult  to  interpret. 

The  geological  history  of  Chicago  has  been  peace- 
ful and  uneventful  to  a  degree  only  occasionally  en- 
countered. There  have  been  here  no  volcanic  erup- 
tions, no  formation  and  destruction  of  mountains  and, 
in  short,  no  geological  forces  have  here  acted  with  that 
vigor,  common  elsewhere,  which  has  left  traces  in 
broken,  folded  and  contorted  rocks.  It  is  true  that  the 
site  of  the  city  has  many  times  been  submerged  by  the 
sea  and  emerged  again,  but  these  submergences  have 
been  slow  and  orderly  processes  due  either  to  bodily 
changes  in  the  level  of  the  region  as  a  whole  or 
to  variations  in  the  level  of  the  sea.  Certainly  the 
rocks  under  the  city  still  lie  nearly  as  horizontal  as 
they  were  when  they  were  deposited  and  they  have 
undergone  few  changes  save  the  hardening  and  re- 
crystallization  due  to  the  pressure  of  overlying  rock 
and  the  lapse  of  time.     The  changes  of  level  which 

[73] 


2  Field  Museum  of  Natural  History 

have  admitted  and  driven  back  the  sea  have  not  been 
great.  But  once  has  the  region  stood  at  any  great  ele- 
vation, and  at  no  time  have  the  submerging  seas  at- 
tained any  great  depths.  Even  at  the  present  day  a 
depression  of  the  land  levels  of  only  six  hundred  feet 
would  again  submerge  Chicago  beneath  the  sea. 

The  rocks  underlying  the  city  are  those  formed 
wholly  during  Paleozoic  time.  The  word  Palezoic 
means  ancient  life  and  the  Paleozoic  time  or  era  was 
the  time  of  ancient  and  primitive  life.  It  began  when 
evidences  of  life  were  first  abundantly  preserved  in  the 
rocks  and  continued  with  the  gradual  appearance  of 
higher  types  of  life  until  the  time  when  the  reptiles 
represented  the  highest  forms  of  animal  existence. 
Paleozoic  time  is  divided  into  periods  which  correspond 
to  stages  in  this  gradual  development  of  lower  to  high- 
er forms  of  life.  The  opening  period  of  the  Paleozoic 
is  the  Cambrian,  often  called  the  Age  of  Trilobites,  and 
it  is  with  conditions  during  this  period  that  this  history 
begins.  All  the  rocks  under  Chicago  were  formed  dur- 
ing the  earlier  periods  of  Paleozoic  time  before  even 
the  most  primitive  types  of  vertebrate  animals  were 
numerous,  so  that  the  only  fossils  to  be  found  here  are, 
save  for  a  few  fish  teeth,  those  of  invertebrates  and 
some  obscure  forms  of  primitive  plants.  These  Pale- 
ozoic rocks  must  necessarily  rest  upon  still  older  rocks 
but  these  are  so  deeply  buried  here  that  even  the  deep- 
est artesian  wells  of  the  city  have  not  penetrated  them. 

During  the  whole  of  Paleozoic  time,  the  great  mass 
of  the  North  American  Continent  was  in  existence.  It 
seems  to  have  had  much  the  same  form  that  it  now  has. 
Highlands  to  the  east  and  west  were  separated  then 
as  now  by  a  broad,  low,  central  valley.  But  as  the 
land  stood  at  a  lower  elevation  than  at  present,  this 
great  central  valley  was  submerged  in  a  shallow  in- 
terior sea  of  vast  extent.    This  sea  divided  the  land  of 

[74] 


LEAFLET  7. 


rm 


# 


mwMxm    &msm 


MAP  OF  NORTH  AMERICA  IN  EARLY  CAMBRIAN  TIME. 
AFTER  SCHUCHERT. 


Early  Geological  History  op  Chicago  3 

the  continent  into  a  number  of  great  islands  which 
have  been  mapped  and  named.  The  interior  sea  was 
constantly  encroaching  upon  the  lands  and  retreating 
from  them.  The  site  of  Chicago  was  therefore  alter- 
nately sea  bottom  and  dry  land.  When  not  submerged 
by  the  sea  it  was  usually  a  part  of  the  great  northern 
land  called  the  Canadian  Shield. 

Chicago  in  Early  Cambrian  Time 

At  the  beginning  of  Paleozoic  time  and  of  the 
Cambrian  period  the  site  of  Chicago  had  long  been  a 
land  with  a  warm  and  equable  climate.  This  land  was 
part  of  a  great  continent  (Plate  I),  larger  than  the 
North  America  of  today.  At  times  it  was  wider  and 
at  times  somewhat  narrower  than  the  present  conti- 
nent. To  the  southwest  it  extended  through  Mexico 
into  the  Pacific  and  to  the  northeast  it  extended 
through  Greenland  and  Iceland  into  Europe.  Chi- 
cago1 occupied  a  geographical  position  in  this  land 
much  the  same  as  it  occupies  in  North  America  to-day. 
It  was  centrally  located,  nearer  the  Atlantic  than  the 
Pacific  and  probably  in  a  great  central  valley  corre- 
sponding to  the  Mississippi  valley  of  to-day.  Prob- 
ably there  were  no  great  lakes  in  the  vicinity  and  we 
know  nothing  of  the  water  courses  of  that  time.  Noth- 
ing can  be  said  with  assurance  of  the  rainfall  or  the 
amount  of  sunshine.  The  climate  was  warm  and 
equable  and  it  has  been  surmised  that  the  prevailing 
winds  were  from  the  east.  There  were  no  mountains 
or  high  hills  near.  The  air  contained  oxygen,  but  how 
much  we  do  not  know.  The  whole  region  was  prob- 
ably covered  with  loose,  drifting  sands.    It  was  a  place 


1  By  Chicago  is  meant  the  geographical  position  now  occu- 
pied by  the  city.  The  same  is  to  be  understood  of  other  place 
references  except  those  referring  to  the  Oceans  which  although 
varying  greatly  in  size  and  shape  have  always  retained  their 
identities. 

[75] 


4  Field  Museum  op  Natural  History 

of  moving  dunes  devoid  of  vegetation.  Whether  it  was 
completely  devoid  of  vegetation  and  terrestrial  forms 
of  life  is  not  known  but  no  traces  of  such  have  ever 
been  found.  The  presence  of  oxygen  in  the  air  proves 
that  vegetation  existed  somewhere  and  the  presence  of 
sea  weeds  in  the  sea  is  known.  It  is  also  probable  that 
the  rivers  and  ponds  held  algae  and  other  primitive 
forms  of  vegetation.  No  woody  plants  yet  existed.  The 
streams  apparently  were  inhabited  by  unknown  forms 
of  soft-bodied  animals  which  have  left  no  traces  of 
their  existence.  At  least,  the  fishes  when  they  made 
their  first  appearance  at  a  much  later  date  seem  to 
have  originated  in  fresh  water.  The  Eurypterid,  a 
scorpion-like  animal  from  which  the  scorpion  and  spid- 
ers have  descended,  may  have  inhabited  the  streams. 

Chicago  in  Late  Cambrian  Time 

During  Early  Cambrian  time  the  sea  began  to  en- 
croach upon  the  land  until  after  the  middle  of  the 
period,  Chicago  was  first  upon  the  sea-shore  and  later 
was  submerged  by  a  shallow  interior  sea  which  finally 
reached  nearly  or  quite  to  the  northern  boundary  of 
Lake  Michigan.     (Plate  II.) 

The  sands  of  this  sea-shore  and  sea  bottom  are 
now  found  as  a  sandstone  which  has  been  penetrated 
by  artesian  wells  at  depths  around  fifteen  hundred 
feet.  Not  much  can  be  learned  from  the  chips  of  sand- 
stone brought  up  by  the  drilling  operations,  but  this 
bed  of  sandstone  appears  at  the  surface  near  the  La- 
croix  river  in  Wisconsin  and  it  has  been  studied  there. 
The  nearest  land  was  the  great  mass  of  the  Canadian 
Shield  to  the  north  and  there  were  other  lands  to  the 
east,  southeast  and  northwest.  The  sea  was  shallow 
and  the  water  was  salt.  Probably  the  saltness  dif- 
fered in  quality  and  degree  from  that  of  the  modern 
ocean.    It  was  warm,  for  the  climate  remained  mild. 

[76] 


LEAFLET  7. 


PLATE  II. 


MAP  OF  NORTH  AMERICA  IN  LATE  CAMBRIAN  TIME. 
AFTER  SCHUCHERT. 


Early  Geological  History  of  Chicago  6 

The  shores  were  bordered  with  sand  dunes,  the  wastes 
from  which  added  to  the  thickness  of  the  sand  on  the 
sea  bottom.  There  were  no  fish,  for  fish  had  not  yet 
appeared  on  earth,  nor  were  there  any  other  vertebrate 
animals.  There  may  have  been  jelly-fish,  for  traces 
of  them  have  been  found  in  other  parts  of  the  world 
in  rocks  of  the  period.  If  there  were  other  swimming 
creatures  they  must  have  possessed  soft  bodies  which 
did  not  become  fossilized.  In  other  respects  the  as- 
pect was  not  in  general  different  from  that  of  the 
sandy  beaches  and  shallow  waters  of  the  present  day, 
although  the  details  were  altogether  strange.  The 
abundant  forms  of  life  were  worms,  lampshells  or 
brachiopods  and  the  crab-like  trilobites. 

The  worms  were  very  abundant,  for  their  burrows 
and  trails  over  the  sand  are  numerous.    As  they  were 


Fig.  1. 
A  worm  of  Cambrian  time.    Drawn  by  Carl  F.  Groncmann. 

soft-bodied  creatures,  they  have  not  fossilized,  so  that 
we  know  little  of  them  except  that  they  were  numer- 
ous. A  few  impressions  from  their  bodies  left  in  the 
mud  of  other  seas  of  the  time  show  that  some  of  them 
(Fig.  1)  were  equipped,  centipede-fashion,  with  num- 
erous legs  and  a  few  other  features  are  obscurely  in- 
dicated. The  absence  of  fish  or  others  of  the  larger, 
predatory  animals  seems  to  have  permitted  the  worms 
to  increase  until  they  were  a  far  more  important  ele- 
ment in  the  life  of  the  Cambrian  beaches  than  they 
are  on  modern  shores.  The  lampshells  were  bivalved 
shells  evolved  from  the  worms  of  an  earlier  period. 
There  were  many  of  them  and  they  often  grew  in 

[77] 


6 


Field  Museum  of  Natural  History 


colonies.  They  were  smaller  than  the  common  clam 
shell  of  the  present  and  lived  half  buried  in  sand. 
They  were  more  or  less  fixed  in  position  by  long  muscu- 
lar stems  or  peduncles  which,  buried  in  sand,  acted  as 
anchors.  The  peduncle  also  served  to  pull  the  animal 
completely  under  the  sand  in  time  of  danger.  It  is 
very  remarkable  that  one  of  these  forms,  the  Lingula, 
has  survived  to  the  present  time  with  only  insignificant 
changes  in  form  or  structure.     (Fig.  2.) 


Fig.  2. 

a.  A  recent  brachiopod. 

b.  Shell  of  a  Cambrian  brachiopod. 


Fig.  3 
A  Cambrian  trilobite. 


Most  of  the  other  orders  of  shelled  animals  were 
represented  by  an  occasional  individual.  The  trilo- 
bites  (Fig.  3),  curious  crab-like  creatures  which 
crawled  over  the  sand,  were  the  dominant  animals  of 
the  period.  They  had  lobed  and  jointed  shells.  While 
they  resembled  crabs  they  are  not  related  to  them  nor 
are  they  closely  related  to  any  modern  animal.  It  is 
thought  by  many  that,  at  a  later  time,  insects  descended 
from  them.    They  were  at  this  time  small,  few  meas- 


[78] 


Early  Geological  History  of  Chicago  7 

uring  more  than  an  inch  and  a  half  in  length.  There 
were  other  shelled  animals  which  were  not  common 
near  Chicago  and  of  course  the  life  of  the  deeper  seas, 
such  as  corals,  was  entirely  absent  here.  Besides  these 
there  were  numerous  unknown  animals  with  bodies 
which  had  no  hard  parts  to  fossilize.  These  have  left 
no  traces  of  their  existence  except  an  occasional  track 
on  the  sand.  Vegetation  was  confined  to  the  sea-weeds. 
Beyond  the  fact  that  many  of  them,  like  the  present 
sea-weeds  were  algae,  little  is  known  of  them.  They 
must  have  been  numerous  to  provide  food  and  oxygen 
for  the  abundant  animal  life. 

Chicago  in  Early  Ordovician  Time 
The  second  period  of  Paleozoic  time  was  the  Ordo- 
vician, formerly  called  the  Lower  Silurian.  It  was  the 
time  when  cephalopods,  a  group  of  animals  of  which 
the  nautilus,  squid  and  octopus  are  the  best  known 
modern  forms,  first  became  prominent  features  in  the 
life  of  the  sea.  Around  Chicago  there  was  a  gradual 
transition  from  the  Cambrian  to  Early  Ordovician 
time.  There  was  no  abrupt  change  of  any  kind,  but 
Cambrian  forms  of  life  were  gradually  superseded  by 
the  higher  forms  which  characterize  the  Ordovician. 
The  rocks  of  the  transition  period  have  not  preserved 
their  fossils  well,  so  that  not  much  can  be  said  of  the 
life  of  the  time,  beyond  that  it  was  intermediate  in 
character  between  that  of  the  Cambrian  and  that  of  the 
later  Ordovician.  The  region  was  still  the  bottom  of  a 
shallow  sea  of  fluctuating  depth.  The  bottom  was  pre- 
dominantly sandy.  The  water  was  clear,  for  at  no  time 
was  the  bottom  muddy.  The  sandy  bottom  of  the  Cam- 
brian sea  had,  as  Ordovician  time  opened,  become 
limey  from  an  accumulation  of  the  shells  of  small  ani- 
mals which  inhabited  the  sea.  This  change  was  due, 
in  part  at  least,  to  the  distance  from  shore  which  in- 
creased with  the  continued  advance  of  the  sea.    This 

[79] 


8  Field  Museum  op  Natural  History 

condition,  which  persisted  well  into  the  Early  Ordo- 
vician,  has  left  a  record  in  the  form  of  a  limestone 
called  in  older  descriptions  the  Lower  Magnesian  Lime- 
stone. In  later  descriptions  it  is  sometimes  called  the 
Prairie  du  Chien  limestone.  This  limestone  was 
formed  from  the  shells  and  other  limey  parts  of  the 
marine  animals  of  the  time.  Under  the  city  it  has  been 
penetrated  by  drills  at  depths  slightly  in  excess  of  one 
thousand  feet.  A  thickness  of  from  160  to  450  feet 
may  give  some  hint  of  the  long  duration  of  this  sea. 

This  great  advance  of  the  sea,  which  began  in 
Middle  Cambrian  time,  continued  until  in  the  first 
part  of  the  Early  Ordovician,  it  covered  most  of  the 
interior  of  the  North  American  continent.  (Plate  III.) 
It  was  followed  by  an  equally  great  recession,  which 
long  before  the  end  of  the  Lower  Ordovician  left  Chi- 
cago once  again  in  the  interior  of  a  great  continent. 
(Plate  VI.)  Conditions  were  much  like  those  of  the 
earlier  emergence  both  physically  and  geographically. 
The  climate  was  arid.  The  surface  was  covered  with 
moving  sand  dunes.  The  life  in  the  streams  and  rivers 
must  have  been  of  a  more  advanced  type  than  before, 
although  no  traces  of  such  life  remain.  The  streams 
may  have  held  a  primitive  type  of  fish.  Some  fish 
remains  have  been  discovered  in  deposits  of  this  age 
in  the  Rocky  Mountains.  Artesian  wells  in  Chicago 
and  Cook  County  penetrate  at  depths  of  around  800 
to  1000  feet,  a  coarse,  white  sandstone  which  is  in 
places  one  hundred  and  fifty  feet  thick.  This,  the 
St.  Peter's  sandstone,  is  the  sand  of  the  dunes  of  this 
old  land  solidified  into  rock.  It  does  not  appear  at  the 
surface  in  the  City  of  Chicago  but  may  be  seen  along 
the  Illinois  River  near  Ottawa  and  in  Deer  Park.  The 
site  of  Chicago  remained  land  until  the  beginning  of 
Mid-Ordovician  time  when  it  was  submerged  by  a  new 
transgression  of  the  sea. 

[80] 


LEAFLET  7. 


PLATE  III. 


MAP  OF  NORTH  AMERICA  IN  EARLY  ORDOVICIAN  TIME. 

AFTER  GRABAU. 


Early  Geological  History  of  Chicago  9 

Chicago  in  Mid-Ordovician  Time 

Mid-Ordovician  time  was  of  long  duration,  with 
much  oscillation  between  land  and  sea.  It  was  inaug- 
urated in  this  vicinity  by  a  great  advance  of  the  sea. 
A  shallow  interior  sea  extending  north  from  the  Gulf 
of  Mexico  soon  submerged  the  sand  dunes  of  the  earlier 
land.  This  was  followed  by  a  great  inundation  from 
the  Arctic  Ocean,  which  attained  such  vast  propor- 
tions that  little  beyond  a  few  large  islands  remained 
of  the  North  American  continent  (Plate  VII) .  Finally, 
before  the  end  of  the  Mid-Ordovician,  nearly  all  the 
marine  water  was  withdrawn  and  Chicago  was  again 
on  land.  This  sea  was  a  shallow,  clear-water  sea  much 
like  the  preceding  ones,  but  probably  deeper.  The 
waters,  even  those  of  the  extension  of  the  Arctic 
Ocean,  were  warm.  These  waters  swarmed  with  in- 
vertebrate life  which  resembled  that  of  the  preceding 
epoch  as  well  as  that  of  the  Niagara  to  be  later  de- 
scribed. The  rocks  deposited  in  this  sea,  called  the 
Galena-Trenton  formation,  reach  the  surface  nowhere 
in  Chicago  or  Cook  County.  They  lie  from  500  to  600 
feet  beneath  the  surface  and  are  from  270  to  390  feet 
in  thickness.  They  are  cream-colored  or  yellowish 
magnesian  limestones. 

Chicago  in  Late  Ordovician  Time 

The  withdrawal  of  the  sea  at  the  close  of  the  Mid- 
Ordovician  left  Chicago  upon  the  western  shore  of  a 
great  interior  sea  connected  with  the  Gulf  of  Mexico 
and  upon  land  which  was  part  of  a  continent  greater 
than  the  North  America  of  the  present  day.  Of  con- 
ditions in  the  territory  near  the  city  we  know  little 
beyond  the  facts  that  there  were  no  mountains  and 
that  the  climate  remained  mild.  This  land  did  not 
persist  long,  for  soon  a  new  cycle  of  water  movement 
began  and  the  seas  advancing  northward  from  the 

[81] 


10  Field  Museum  of  Natural  History 

Gulf  of  Mexico  soon  resubmerged  the  site  of  the  city  in 
muddy  waters.  These  waters  were  muddy  because 
elevations  of  the  land  to  the  east  and  south  had  at- 
tained sufficient  magnitude  to  increase  the  current  of 
the  rivers,  which  now  carried  large  quantities  of  mud 
to  the  interior  sea.  This  state  of  affairs  continued 
until  the  site  of  Chicago  was  covered  so  deeply  with 
mud  that  now  when  it  is  compressed  to  rock  it  covers 
the  older  rocks  under  the  city  to  a  depth  of  from  100 
to  250  feet.  This  rock,  formed  from  the  consolidated 
muds  of  the  Late  Ordovician  seas,  is  a  shale  mixed 
with  limestone.  It  was  formerly  called  by  some  the 
Cincinnati  shale  and  by  others  the  Hudson  River  shale. 
It  is  now  generally  referred  to  as  the  Maquoketa  shale. 
It  underlies  the  present  rock  surface  at  depths  of  from 
250  to  400  feet.  Before  the  close  of  the  Ordovician 
the  sea  again  withdrew  for  a  considerable  period  leav- 
ing the  site  of  the  city  again  land.  Nothing  is  known 
of  the  climate,  geography  or  life  of  this  land. 

Chicago  in.  Silurian  Time 

The  Silurian  division  of  Paleozoic  time  followed 
the  Ordovician.1  The  Silurian  is  separated  from  the 
Ordovician  more  by  changes  in  the  relation  of  land  and 
sea  areas  than  by  any  radical  change  in  the  character 
of  life.  For,  while  nearly  all  the  genera  and  species 
of  animal  life  in  the  Silurian  were  new,  they  were 
members  of  classes  and  families  of  animals  that  were 
already  common  in  Ordovician  time.  The  beginning 
of  Silurian  time  found  Chicago  on  a  land  area,  where 
it  had  been  left  by  the  retreating  seas  of  the  Ordo- 
vician. The  records  of  early  Silurian  time  are,  in  this 
vicinity,  somewhat  difficult  of  interpretation,  but  ap- 


1  The  older  geologists  grouped  the  Ordovician  and  Silurian 
into  one  period  which  they  called  the  Silurian.  The  Ordovician 
was  then  the  Lower  Silurian  and  the  period  now  known  as  the 
Silurian  was  called  the  Upper  Silurian.- 

[82] 


LEAFLET  7. 


•# 


6 

COMMON  FOSSILS  FROM  THE  NIAGARA  LIMESTONE  OF  CHICAGO. 
1,   CORAL,   CLADOPORA.       2,   COILED   CEPHALOPOD,    NAUTILUS.       3,   CORAL,    DIPHYPHYL- 
4,   CUP  CORAL,   OMPHYMA.      5,   COMPOUND  CORAL,    FAVOSITES. 
6,   STRAIGHT  CEPHALOPOD,   ORTHOCERAS. 


LUM. 


Early  Geological  History  of  Chicago  11 

parently  the  site  of  the  city  remained  land  during  the 
first  part  of  the  Silurian.  This  was  a  time  of  advanc- 
ing seas  and  diminishing  lands  so  that  by  Mid-Silurian 
time  Chicago  was  again  submerged  by  an  interior  sea. 
This  Mid-Silurian  time  is  called  the  Niagara  epoch. 
This  name  is  given  the  epoch  because  rocks  formed 
in  the  seas  of  the  time  are  exposed  near  Niagara  Falls 
and  were  studied  there.  It  has  nothing  to  do  with  the 
falls  and  river,  which  did  not  come  into  existence  until 
ages  later. 

For  some  time  past  there  had  been  important 
changes  in  the  elevation  of  many  parts  of  the  conti- 
nent. Far  to  the  east  in  New  England,  a  range  of 
mountains  (the  Taconic  Mountains)  appeared  and 
these  shut  off  access  of  the  sea  from  the  east.  To  the 
southeast  was  the  Cincinnati  uplift,  where  great  areas 
had  been  raised  above  the  general  surface,  and  to  the 
south  and  southwest  there  was  another  elevated  region, 
the  Ozark  uplift.  These  highlands  blocked  access  of 
the  sea  from  the  East,  South  and  West  and  therefore 
Silurian  transgression  of  the  sea  which  submerged 
Chicago  came  from  the  Arctic. 

When  this  invasion  of  the  waters  of  the  Arctic 
was  at  its  height,  Chicago  was  on  the  floor  of  a  great 
interior  sea  which  covered  the  northern  part  of  the 
Mississippi  valley.  (Plate  VII.)  To  the  north  it  was 
open  to  the  Arctic  Ocean  and  to  the  south  it  at  one 
time  joined  another  sea  which  in  turn  opened  on  an 
eastward  extension  of  the  Pacific  Ocean.  The  nearest 
land,  Mississippia,  a  great  land  mass  to  the  west,  occu- 
pied what  is  now  the  west  central  part  of  the  country 
and  included  parts  of  Canada  and  Mexico.  Mississip- 
pia was  separated  by  a  narrow  sea  from  Cascadia, 
which  was  a  relatively  long  and  narrow  strip  of  land 
along  the  western  border  of  the  Continent.  To  the 
east  and  southeast  another  great  land  mass,  Appala- 

[83] 


12  Field  Museum  of  Natural  History 

chia,  included  what  is  now  the  eastern  border  of  the 
Continent.  To  the  northeast  and  more  distant  were 
the  shores  of  Atlantica,  which  extended  as  far  south 
as  the  St.  Lawrence  valley  and  covered  parts  of  east- 
ern Canada,  Greenland  and  some  of  Europe.  Exact 
boundaries  of  these  lands  and  seas  are  not  yet  definite- 
ly known  and  maps  of  them  are  constantly  altered  as 
the  result  of  continued  study.  Although  this  sea  was 
an  arm  of  the  Arctic  Ocean,  its  waters  were  warm  and 
semi-tropical  conditions  prevailed.  The  waters  were 
clear  and  swarming  with  life. 

As  the  shallow  Arctic  seas  with  their  warm  waters 
afforded  a  ready  path  of  communication  with  the  north 
of  Europe,  the  Silurian  fauna  of  the  Chicago  area  had 
a  decidedly  European  aspect.  The  great  land  of  Ap- 
palachia  to  the  east  interposed  a  barrier  to  the  migra- 
tion of  marine  animals  between  the  interior  sea  and 
the  Atlantic  coast  and  no  intermingling  of  the  life 
of  these  two  regions  was  possible.  Consequently  the 
life  of  the  Silurian  seas  of  Chicago  was  more  like  that 
of  Europe  than  like  that  of  the  nearer  eastern  part  of 
the  United  States. 

This  was  a  coral  sea.  Primitive  forms  of  corals 
had  existed  even  in  early  Cambrian  time  but  the  early 
seas  of  Chicago  were  seldom  favorable  to  their  growth. 
By  the  middle  of  the  Ordovician,  corals  had  become 
numerous  and  had  separated  into  many  varieties.  In 
the  Niagaran  seas  of  Silurian  time,  corals  had  become 
even  more  numerous  and  existed  in  even  greater  va- 
riety. The  prominence  of  the  corals  was  in  fact  one 
of  the  notable  features  of  these  Silurian  seas.  As  the 
sea  in  the  vicinity  of  Chicago  was  especially  favorable 
to  coral  growth,  these  animals  were  an  important  fea- 
ture of  these  waters.  The  coral  is  a  simple  form  of 
animal.  Its  body  is  little  more  than  a  hollow  cylinder 
of  flesh.    A  mouth  at  the  upper  end  is  surrounded  by 

[84] 


Early  Geological  History  of  Chicago 


18 


a  crown  of  tentacles.  The  animal  rests  upon  a  skele- 
ton or  pedestal  of  carbonate  of  lime  which  is  secreted 
by  the  animal  itself.    This  pedestal  is  either  cylindrical 


Fig.  4. 

a.  Fossil  of  a  cup  coral  of  Niagara  time.      b.  Cup  coral  of  Niagara  time. 

Soft  parts  according  to  Fen  ton. 

or  conical.  Conical  forms  are  often  curved  so  that 
they  resemble  blunt  horns.  There  were  simple  indi- 
vidual corals  (Fig.  4)  each  of  which  grew  by  itself  on 


Fig.  6. 
A  honeycomb  (compound)  coral  of  Niagara  time. 

its  own  pedestal  and  there  were  compound  forms  in 
which  the  stony  bases  or  pedestals  were  joined  in  one 
mass.     (Fig.  5.)     This  mass  commonly  took  either  a 

[85] 


14  Field  Museum  op  Natural  History 

hemispherical  or  a  dome-like  form  or  grew  as  clusters 
of  branching  arms  which  roughly  resembled  branch- 
ing trees.  These  colonies  attained,  under  favorable 
conditions,  considerable  size.  Some  of  the  coral  domes 
measured  as  much  as  fourteen  feet  across  and  a  height 
of  fifteen  feet  for  branching  forms  has  been  recorded. 
Although  the  colonies  attained  considerable  size  the 
individual  corals  remained  small.  Usually  they  were 
less  than  a  quarter  of  an  inch  in  diameter,  although 
there  are  some  which  measured  more  than  an  inch. 

In  these  Silurian  seas  the  first  coral  reefs  ap- 
peared, for  at  this  time  the  coral  first  acquired  the 
reef  building  habit.  Although  the  group  to  which 
modern  corals  belong  had  not  yet  developed,  there  is 
no  reason  to  suppose  that  the  appearance  of  a  Silurian 
coral  bed  was  in  essential  features  different  from  that 
of  a  modern  coral  reef.  (Fig.  6.)  On  a  base  com- 
posed of  the  stony  skeletons  of  past  generations,  great 
dome-like  masses  of  coral  mingled  with  branching  tree- 
like forms  and  all  were  crowded  together.  Resemb- 
lance to  modern  coral  beds  was  heightened  by  the 
presence  of  many  forms  of  the  more  delicate  incrusting 
and  branching  bryozoa.  These  bryozoa  were  coral- 
like animals  which  grew  in  colonies  of  minute  individ- 
uals, grouped  like  the  compound  corals  on  supports  of 
carbonate  of  lime  which  the  animals  secreted.  These 
supports  resembled  in  a  general  way  the  branching 
corals,  but  they  were  commonly  more  slender  and  in- 
clined to  grow  together  into  intricate  network  pat- 
terns. Many  grew  in  fan  or  leaf  form,  others  were 
netted  or  branching  threads  incrusting  corals  or  other 
solid  supports. 

Swarming  over  the  sea  bottom  in  great  abundance 
were  the  crinoids,  strange,  flower-like  creatures  utterly 
unlike  in  appearance  any  animal  commonly  encoun- 
tered at  the  present  day,  although  a  few  species  still 

[86] 


Early  Geological  History  op  Chicago 


15 


£ 


[87] 


16  Field  Museum  of  Natural  History 

exist  in  tropical  seas.  They  were  attached  to  the  sea 
bottom  by  long  stems  and  their  small,  round  bodies 
were  provided  with  many  long,  feathery  arms.  Their 
appearance  was  so  flower-like  that  they  are  often  called 
stone  lilies.  They  were  gregarious  in  habit  and  in 
parts  of  the  Niagara  sea  at  Chicago  they  were  so 
numerous  that  these  places  have  been  described  as 
"veritable  flower  beds  of  stone  lilies  where  beautiful 
and  varied  forms  grew  in  groves,  as  it  were."  Al- 
though they  do  not  look  at  all  like  them,  they  are  actu- 
ally closely  related  to  the  starfish.  They  are  somewhat 
like  a  starfish  turned  upside  down  and  perched  on  the 
top  of  a  long,  jointed  stem  and  provided  with  many, 
long,  feathery  arms  in  place  of  the  five  short  rays  of 
the  starfish.  The  stem  is  intended  to  keep  the  animal 
out  of  the  mud  and  to  anchor  it  so  that  it  cannot  drift 
away  into  places  where  food  supply  and  other  living 
conditions  might  be  unsuitable.  This  stem  is  built  up 
from  many  short  discs  or  joints.  These  joints  are  of 
a  stony  material,  a  mixture  of  the  carbonates  of  lime 
and  magnesia.  The  stem  is  terminated  below  by 
branching  roots  or  by  a  kind  of  flat  foot  which  anchors 
it  to  its  support.  On  the  top  of  the  stem  is  the  body, 
which  is  in  many  species,  of  the  size  and  shape  of  a 
walnut.  This  body  is  inclosed  in  a  jointed  armor  of 
limestone  plates,  which  is  called  the  cup.  From  the 
top  of  the  body  grow  the  jointed  arms  which  wave  in 
the  water  and  direct  food  to  the  mouth.  (Fig.  7.) 
There  were  many  kinds  of  crinoids,  some  having  long, 
feathery,  branching  arms,  while  others  were  simpler 
and  less  conspicuous.  Blastids  grew  with  the  crinoids. 
These  were  much  like  the  crinoids  and  grew  on  stems 
but  had  no  arms. 

Although  crinoids  and  corals  were  the  most  num- 
erous inhabitants  of  this  sea,  other  animals  were  by 
no  means  rare.    The  brachiopods  were  animals  cov- 

[88] 


Early  Geological  History  of  Chicago 


17 


Fig.  7.      A  crinoid  of  Niagara  time.     Drawn  by  Carl  F.  Gronemann. 
[89] 


18  Field  Museum  op  Natural  History 

ered  by  shells  which  at  first  sight  appear  somewhat 
like  clam  shells,  although  these  animals  are  no  rela- 
tives of  the  clam.  Brachiopods  were  anchored  to  the 
bottom  much  as  the  common  mussel  shell  of  the  pres- 
ent day  seas  is,  by  a  long,  slender  thread. 

Sponges  were  not  uncommon,  but  they  were  not 
like  the  common  sponge  of  the  present  day.  The 
sponge  was  an  animal  with  a  jelly-like,  hollow  body, 
supported  by  a  stiff,  porous  skeleton.  In  the  valuable 
varieties  of  modern  sponge,  this  skeleton  is  composed 
of  horny  matter,  but  these  ancient  sponges  had  skele- 
tons composed  of  the  mineral  silica.  These  silica  skele- 
tons were  sometimes  in  the  form  of  a  continuous,  por- 
ous network  like  the  horny  skeleton  of  modern,  com- 
mercial sponge  and  sometimes  they  were  disconnected, 
minute  rods  and  spicules.  It  is  probable  that  sponges 
with  horny  skeletons  also  lived  at  this  time,  but  as 
such  varieties  had  no  hard  parts,  no  trace  of  them 
remains. 

Not  all  the  inhabitants  of  this  sea  were,  like  those 
already  mentioned,  anchored  to  the  sea  bottom.  There 
were  crawling  and  swimming  forms  as  well.  The  tril- 
obites,  dominant  animals  of  the  Cambrian  seas  were 
still  numerous,  though  not  as  important  as  they  had 
been.  They  had  the  habits  and  much  of  the  appear- 
ance of  crabs.  Like  crabs  they  were  scavengers,  eat- 
ing carrion.  They  did  not  have  the  long,  spreading 
legs  of  the  crab  but  moved  on  numerous  short,  jointed 
legs  attached  to  the  underpart  of  the  body,  either 
crawling  around  on  the  bottom  or  skimming  along  just 
above  it.  Like  the  crab  they  relied  for  protection  on 
a  horny  shell,  which  however  did  not  cover  the  under 
side  of  the  body.  When  threatened  by  an  enemy,  most 
Silurian  trilobites  coiled  themselves  into  a  compact 
ball  which  presented  on  all  sides  an  unbroken  armor 
of  horny  shell.    The  commoner  trilobites  of  the  Chi- 

[90] 


Early  Geological  History  of  Chicago 


19 


cago  seas  were  about  an  inch  to  two  inches  long.  They 
were  oval  in  outline  and  the  shell  consisted  of  three 
parts.  A  front  end,  the  head,  was  connected  by  a 
many- jointed  thorax  to  the  rear  end,  the  abdomen. 
(Fig.  8.)  Although  so  crab-like  in  habits  and  appear- 
ance, the  trilobite  was  no  relative  of  the  crab,  but  be- 
longed to  a  group  of  animals  which  have  long  been 
extinct. 

The  dominant  animals  of  the  Chicago  seas  at  that 
time  were  the  cephalopods.  They  are  called  the  domi- 
nant animals,  not  because  they  were  the  most  numer- 
ous, for  they  were  not,  but  because  some  of  them 


d 


Fig.  8. 

Trilobites  of  Niagara  time.     Drawn  by  Carl  F.  Gronemann. 

a.  Extended.      b.  Coiled. 

were  large  and  they  were  the  most  highly  developed 
form  of  animal  life  then  existing  in  any  considerable 
numbers.  In  modern  seas,  the  cephalopods  are  repre- 
sented by  only  a  few  forms,  of  which  the  octopus,  squid 
and  nautilus  are  the  best  known.  In  Silurian  seas 
there  was  a  great  variety  of  these  animals  and  they 
were  provided  with  large,  strong  shells.  The  common 
cephalopod  of  the  Chicago  Silurian  seas  was  the  Or- 
thoceras.  (Fig.  9.)  This  mollusk  had  a  long,  perfectly 
straight,  spear-like  shell.  This  shell  gradually  tapered 
from  a  cup  in  front  inhabited  by  the  animal  to  a  point 


[91] 


20  Field  Museum  of  Natural  History 

at  the  rear.  The  shell  was  divided  into  segments.  As 
the  animal  outgrew  its  shell,  instead  of  moulting,  as 
trilobites  did,  it  grew  a  new  and  larger  cup  in  front 
of  the  old  one  and  moved  into  it  and  for  the  rest  of  its 
life  it  dragged  its  old,  abandoned  residence  along  be- 


Fig.  9. 
An  orthoceras  of  Niagara  time.     Drawn  by  Carl  F,  Gronemann. 

hind  it.  The  animal  itself  was  much  like  its  relative, 
the  octopus.  It  had  a  mouth  in  front,  surrounded  by  a 
ring  of  long,  fleshy  tentacles  which  served  both  as 
grasping  arms  and  as  feet.    Like  the  octopus  of  the 

[92] 


Early  Geological  History  of  Chicago  21 

present  day,  it  crawled  on  these  tentacles  and  prob- 
ably it  could  swim  as  well.  It  was  carnivorous  and  fed 
on  smaller  animals.  It  was  variable  in  size.  Many  of 
the  fossils  of  this  animal  found  in  Chicago  are  from 
one  to  three  inches  in  diameter  and  represent  shells 
that  when  complete  were  several  feet  long.  One  speci- 
men in  the  Museum,  of  local  origin,  is  six  inches  in 
diameter  and  over  two  feet  long  and  yet  it  is  only  part 
of  a  shell.  Specimens  have  been  found  elsewhere  eight 
to  ten  inches  in  diameter  and  fifteen  feet  long.  These 
long,  straight  shells  must  have  been  cumbersome  to 
carry  around.  Snails  and  most  other  animals  both  an- 
cient and  modern  that  inhabit  long  shells  have  the 
shell  coiled  so  that  it  occupies  less  space.  Cephalopods 
of  later  ages  inhabited  coiled  shells  and  in  these  Silu- 
rian seas  there  were,  besides  the  straight  shelled  or- 
thoceras,  other  animals  of  the  kind,  some  with  partially 
coiled  shells  and  a  few  with  shells  completely  coiled. 

There  were  other  animals  present  in  decidedly 
lesser  numbers  in  this  part  of  the  Silurian  sea.  There 
were  many  varieties  of  gasteropods,  small,  coiled 
shells  closely  related  to  the  snail  and  an  occasional 
member  of  other  classes  of  the  lower  animals. 

As  animal  life  was  so  plentiful  it  follows  that 
vegetation  must  have  been  equally  abundant  or  these 
animals  could  not  have  found  food.  But  the  vegeta- 
tion, which  was  confined  to  members  of  the  lower  or- 
ders of  plants,  had  no  hard  parts,  so  that  no  trace 
remains  of  the  thick  growths  of  seaweeds  which  must 
have  been  there. 

The  Niagara  Limestone.  Thousands  of  succes- 
sive generations  of  the  animals  just  described  lived 
and  died  in  the  Silurian  sea.  Their  hard  parts,  which 
usually  had  the  composition  of  limestone,  were  prac- 
tically indestructible  and  slowly  accumulated  on  the 
sea  floor.    In  the  course  of  time  they  built  up  a  bed 

[93] 


22  Field  Museum  op  Natural  History 

of  limestone  hundreds  of  feet  thick.  This  is  the  Niag- 
ara limestone  which  underlies  the  whole  city.  All 
quarries,  rock  cuts,  canals  and  tunnels  and  other  rock 
exposures  in  or  near  the  city  are  in  this  Niagara  lime- 
stone. Nowhere  in  Cook  County  have  any  openings 
larger  than  drill  holes  penetrated  through  this  lime- 
stone to  the  older  rocks  below. 

Although  the  hard  parts  of  these  Silurian  animals 
were  practically  indestructible  they  have  proven  to  be 
by  no  means  unchangeable.  Most  of  the  original  fos- 
sil forms  have  been  obliterated  by  the  recrystallization 
of  the  rock  to  a  compact,  crystalline  limestone.  This 
recrystallization  has  not  been  complete,  for  numerous 
fossils,  relics  of  this  ancient  life,  can  be  found  prac- 
tically anywhere  in  this  limestone.  Around  Chicago 
these  fossils  are  not  ordinarily  the  actual  petrified  re- 
mains of  the  animals  but  are  merely  casts  or  impres- 
sions from  which  the  actual  animal  substance  has  long 
since  disappeared.  They  represent  only  the  stony  or 
hard  parts  of  the  animals,  as  the  softer  parts  did  not 
last  long  enough  for  these  casts  to  form.  (Plates  IV 
and  V.) 

Frequently  in  this  limestone  little  nut-like  objects, 
anywhere  from  a  cherry  to  a  walnut  in  size,  may  be 
found.  Some  are  rough  and  plain  of  surface,  others 
are  beautifully  ornamented  with  intricate  designs  of 
raised  and  incised  lines.  These  are  the  bodies  or  cups 
of  crinoids.  In  those  found  near  Chicago  the  arms 
have  not  been  preserved,  but  in  deposits  in  the  same 
sea  in  other  parts  of  the  country,  and  in  particular  near 
Louisville,  Kentucky,  beautiful  specimens  showing 
arms  have  been  secured.  With  the  fossil  crinoid  bodies 
there  are  often  found  numerous  flat  discs,  commonly 
a  quarter  of  an  inch  or  less  in  diameter.  These  are 
sections  of  the  jointed  stems  by  which  the  crinoids 
were  anchored.     Most  of  these  sections  are  circular, 

[94] 


LEAFLET  7. 


COMMON  FOSSILS  FROM  THE  NIAGARA  LIMESTONE  OF  CHICAGO. 

1,  CRINOID  HEADS,    EUCALYPTOCRINUS.      2,  CYSTIDS,   HOLOCYSTITES.      3,  TRILOBITE, 

CALYMENE.     4,   CRINOID  STEMS.     5,    BRACHIOPODS,   SPIRIFER. 

6,   GASTROPOD,    PLEUROTOMARIA. 


Early  Geological,  History  of  Chicago  23 

but  five-sided  or  even  star-shaped  outlines  are  not  un- 
common, and  other  shapes  are  occasionally  found. 

Corals  are,  after  the  crinoids,  the  most  common 
of  the  local  fossils.  Owing  to  the  great  diversity  of 
Silurian  coral  life,  these  fossils  are  present  in  great 
variety.  The  honeycomb  coral  resembles  a  mass  of 
honeycomb  imbedded  in  rock.  Cup  corals  are  often 
found.  A  common  local  form,  the  Zaphrentis,  is  a 
small,  curved  cone  resembling  a  blunt  horn.  Its  larger 
end  is  hollowed  into  a  cup,  which  is  lined  with  numer- 
ous thin  plates  running  from  the  outside  towards  the 
center.  These  fossils  can  seldom  be  broken  from  the 
rock.  What  is  most  frequently  seen  is  a  section  which 
appears  on  a  limestone  surface  as  a  circle  showing  the 
edges  of  numerous  thin  plates  running  from  the  rim 
toward  the  center.  Sometimes  the  section  runs  length- 
wise of  the  coral  so  that  the  outline  of  the  curved, 
horn-shaped  cone  is  visible.  Fragments  of  branching, 
tree-like  corals  are  not  uncommon  and  remnants  of 
other  varieties  are  found  at  times.  The  branching 
forms  of  bryozoa  are  also  frequent.  These  resemble 
branching  corals  but  can  be  distinguished  from  them 
by  a  study  of  their  more  minute  details.  Often  a  net- 
work of  fine,  branching  lines  will  be  found  incrusting 
a  large  fossil.    These  also  are  bryozoa. 

Another  common  fossil  is  the  trilobite.  When 
well  preserved,  as  it  frequently  is,  it  is  a  most  curious 
object.  The  common  variety,  an  inch  or  so  long,  looks 
something  like  a  bug  with  a  rounded  head  and  large 
eyes.  This  head  is  joined  by  a  central  portion  com- 
posed of  many  jointed  rings  to  an  abdomen  of  about 
the  same  size.  The  trilobite  is  sometimes  found  lying 
flat  and  sometimes  coiled  into  a  ball. 

Another  fossil  often  encountered  in  this  limestone 
takes  the  form  of  a  cylindrical,  jointed  rod.  It  is  com- 
monly from  one  to  three  inches  in  diameter,  although 

[96] 


24  Field  Museum  of  Natural  History 

smaller  and  larger  specimens  are  not  infrequent.  A 
complete  individual  may  be  several  feet  long  and  taper 
to  a  point.  Specimens  of  these  have,  on  account  of 
their  size  and  jointing,  been  mistaken  for  fossil  back- 
bones. These  fossils  are  the  shells  of  the  orthoceras, 
the  straight-shelled  cephalopod.  Curved  cephalopod 
shells  are  also  found  but  are  not  common. 

Occasionally  nodules  of  flint,  usually  flat  and  ir- 
regular of  form,  are  encountered.  These  are  the  re- 
mains of  Silurian  sponges.  Other  fossils  are  pres- 
ent in  the  Niagara  limestone  of  Chicago,  but  they  are 
in  lesser  numbers  and  so  they  are  less  likely  to  be 
noticed. 

In  the  Chicago  District  the  Niagara  limestone 
varies  from  two  hundred  and  fifty  to  four  hundred  feet 
in  thickness.  Even  this  is  not  the  original  thickness, 
for  during  the  ages  that  it  has  been  exposed  to  the 
weather,  much  of  it  must  have  eroded  away.  Numer- 
ous quarries  and  outcrops  afford  ample  opportunity 
for  its  study.  Perhaps  the  best  exposure  is  in  the  cut 
of  the  Drainage  Canal  near  Lockport.  (Frontispiece.) 
The  limestone  lies  in  undisturbed,  horizontal  beds  as 
it  was  deposited.  There  are  some  local  gentle  undula- 
tions and  at  Stony  Island  there  is  a  dome  with  steeply 
sloping  sides.  The  origin  of  this  dome  is  somewhat 
of  a  mystery.  It  has  been  suggested  that  it  may  be 
merely  a  mounding  of  limestone  beds  over  an  ancient 
coral  reef,  but  this  is  only  one  of  a  number  of  sug- 
gested reasons  for  its  existence. 

There  are  two  kinds  of  limestone.  One,  typical 
limestone,  is  essentially  a  pure  carbonate  of  lime.  The 
other,  called  dolomite,  is  a  mixture  of  the  carbonates 
of  lime  and  magnesia  in  nearly  equal  parts.  Many 
limestones  are  intermediate  in  composition  between 
these  two.  The  Niagara  limestone  of  the  Chicago  Dis- 
trict is  a  dolomite.    Many  theories  have  been  proposed 

[96] 


MAP  OF  NORTH  AMERICA  AT  THE  END  OF  EARLY  ORDOVICIAN  TIME. 
AFTER  GRABAU. 


Early  Geological  History  of  Chicago  25 

to  account  for  the  presence  of  magnesia  in  such  lime- 
stone as  this,  but  no  one  of  them  has  been  universally 
accepted  as  applying  in  this  instance.  Few  shell-bear- 
ing animals  secrete,  as  was  formerly  supposed,  a  pure 
carbonate  of  lime.  Most  of  them  grow  shells  or  skele- 
tons which  contain  notable  quantities  of  carbonate  of 
magnesia.  It  happens  that  the  crinoids,  the  most 
abundant  animals  of  local  Silurian  seas,  secrete  for 
their  skeletons  a  mixture  unusually  rich  in  carbonate 
of  magnesia.  It  is  quite  possible  that  the  skeletons  of 
the  Silurian  crinoids,  living  as  they  did  under  condi- 
tions unlike  those  of  the  present,  may  have  contained 
magnesia  in  even  greater  quantity.  It  may  well  be 
that  the  magnesia  originally  present  in  the  skeletons 
of  the  Silurian  fauna  is  sufficient  to  account  for  that 
now  present  in  the  limestone.  This,  however,  is  to  be 
taken  as  a  possibility,  not  as  a  certainty.  Asphalt  im- 
pregnates the  limestone  in  places.  It  is  especially  ap- 
parent in  the  limestone  outcrops  at  Windsor  Park  and 
in  the  quarries  at  Thornton.  This  asphalt  is  a  product 
of  the  decay  of  perishable  tissue  of  Silurian  animals. 
Years  ago,  some  of  the  wells  of  the  city  were  contam- 
inated by  petroleum  apparently  derived  from  the  upper 
layers  of  the  limestone.  This  seems  to  have  been  pres- 
ent only  in  small  quantities,  for  it  soon  disappeared. 
It  probably  had  the  same  origin  as  did  the  asphalt. 

The  Niagara  epoch  of  Silurian  time  was  termin- 
ated by  a  withdrawal  of  the  seas  which  had  submerged 
the  continent.  It  is  possible  that  this  withdrawal  did 
not  at  once  bare  the  site  of  the  city,  but  it  is  certain 
that  at  some  period  of  late  Silurian  time  the  city  was 
again  on  land.  Local  records  of  this  time  are  missing, 
as  any  rocks  which  may  have  formed  in  the  seas  of  the 
time  have  long  since  been  destroyed  by  weathering  and 
erosion.  We  may  infer  something  of  local  conditions 
by  a  study  of  better  preserved  records  found  in  other 

[97] 


26  Field  Museum  of  Natural  History 

parts  of  the  country,  but  such  inferences  are  of  real 
value  only  as  they  refer  to  general  conditions.  All  that 
we  may  say  with  assurance  is  that  this  land  was  of 
great  size,  probably  as  large  or  larger  than  the  North 
America  of  the  present  day.  Desert  conditions  existed 
over  much  of  New  York,  Ohio  and  Michigan.  This 
great  desert  may  have  included  Chicago  within  its 
boundaries. 

Chicago  in  Devonian  Time 
The  Devonian  division  of  Paleozoic  time  followed 
the  Silurian.  This  was  the  Age  of  Fishes,  so-called 
because  then  for  the  first  time  fishes  were  an  important 
element  of  the  life  of  the  waters.  When  Devonian  time 
began,  almost  all  of  North  America  had  emerged  from 
the  sea  and  Chicago  was  on  land.  During  the  progress 
of  Devonian  time  there  were  several  oscillations  of  sea 
level.  Several  of  these  may  have  submerged  the  city, 
but  if  so,  any  local  traces  have,  except  in  one  instance, 
long  since  disappeared.  Until  1899  it  was  believed 
that  there  was  no  trace  of  a  Devonian  sea  in  the  Chi- 
cago region,  nor  were  any  Devonian  rocks  known 
nearer  than  those  in  the  vicinity  of  Milwaukee.  In 
1899,  however,  a  few  Devonian  fossils  were  discovered 
in  a  quarry  near  Elmhurst.  This  quarry  is  in  the 
Niagara  limestone.  During  much  of  Devonian  time 
this  was  a  land  area.  The  limestone  formed  in  the 
seas  of  the  earlier  Silurian  time  was  then  surface  rock. 
The  upper  part  had  eroded  away  and  the  numerous 
cracks  which  are  such  a  noticeable  feature  in  this  as 
in  all  other  limestones  had  already  formed.  The  waters 
which  collected  on  the  land  surface  percolated  through 
these  cracks  in  the  limestone  and  enlarged  them.  An 
opening  six  inches  wide  at  the  base  and  sixteen  inches 
high  was  excavated  in  one  place.  At  a  later  period, 
near  the  close  of  the  Devonian,  when  the  sea  again 
covered  this  region,  sand  sifted  from  the  sea  bottom 

[98] 


Early  Geological  History  of  Chicago  27 

into  this  opening  and  with  it  were  carried  shells  and 
and  teeth  of  fishes  which  inhabited  the  sea  thereabout 
(Plate  VII.)  From  this  one,  small  deposit  we  learn, 
first,  that  Chicago  had  been  for  much  of  Devonian  time 
on  land  and,  second,  that  near  the  close  of  Devonian 
time  it  was  again  a  sea  floor.  This  Devonian  sea  was 
not  unlike  the  preceding  one  and  its  invertebrate  life 
was  much  the  same.  Corals  still  abounded  but  there 
were  fewer  crinoids  and  trilobites.  The  principal  dif- 
ference, however,  lay  in  the  presence  of  fish. 

Although  the  Devonian  is  called  the  Age  of  Fishes, 
it  must  not  be  supposed  that  Devonian  seas  were,  like 
modern  oceans,  swarming  with  fish.  Most  Devonian 
fish  were  dwellers  in  fresh  water.  At  that  time  only 
a  few  kinds  had  migrated  into  salt  water.  No  species, 
genera  or  even  families  of  Devonian  fish  have  survived. 
The  great  class  of  bony  fish  to  which  more  than  nine- 
tenths  of  all  modern  species  belong  had  not  then  come 
into  existence.  There  were  sharks  in  the  Devonian 
seas  and  the  ganoid  fishes,  now  represented  only  by 
the  sturgeon,  gar  and  a  few  others,  were  in  Devonian 
times  much  more  important  than  they  are  at  present. 
Nearly  half  of  all  known  species  of  Devonian  fish  be- 
longed to  classes  which  are  now  wholly  extinct.  Many 
of  these  extinct  forms  were  so  unlike  modern  fishes 
that  it  is  questionable  whether  they  should  be  called 
fishes  at  all.  Many  of  them  were  covered  with  jointed 
armor  and  some,  unable  to  swim,  crawled  along  the 
bottom.  As  already  stated,  most  of  them  lived  in  fresh 
water  and,  as  no  record  of  the  land  life  of  Devonian 
time  in  the  Chicago  District  remains,  we  do  not  know 
whether  any  of  these  strange  creatures  ever  inhabited 
this  region.  The  only  fish  that  we  positively  know  to 
have  inhabited  the  seas  of  Devonian  time  here  are  two 
species  of  small  sharks  or  primitive  fish  related  to 
sharks.    We  do  not  know  what  these  shark-like  ani- 

[99] 


28  Field  Museum  of  Natural  History 

mals  looked  like,  for  the  only  parts  that  have  been 
preserved  are  teeth  and  scales.  They  were  probably 
quite  unlike  the  modern  shark  in  appearance.  It  is 
known  that  the  Dinichthys  or  "terrible  fish"  inhabited 
the  eastern  part  of  this  Devonian  sea  and  it  may  have 
been  present  in  the  waters  around  Chicago  as  well. 
The  Dinichthys  were  large,  heavily  armored,  swim- 
ming fish.  Some  of  them  were  over  twenty  feet  long. 
More  than  forty  species  of  smaller  relatives  of  this 
gigantic  fish  inhabited  the  North  American  seas  of  De- 
vonian time. 

All  records  of  the  geological  history  of  Chicago 
from  the  close  of  Devonian  time  to  the  opening  of  the 
glacial  period  in,  geologically  speaking,  almost  recent 
time,  have  been  destroyed.  We  know  from  a  consid- 
eration of  the  known  history  of  other  parts  of  the 
country,  that  submergences  under  the  sea  and  subse- 
quent emergences  of  land  continued,  but  gradually  the 
submergences  became  fewer  and  shorter  and  the  emer- 
gent periods  longer,  so  that  this  later  unknown  part  of 
the  history  would  be,  if  known,  more  and  more  a  record 
of  events  upon  land.  The  rocks  in  which  the  records  of 
these  ancient  times  are  preserved  are,  with  few  excep- 
tions, formed  on  the  sea  floor.  When  the  marine 
waters  are  withdrawn,  these  rocks  become  a  part  of 
the  land.  The  surface  layers  are  then  constantly  ex- 
posed to  such  destructive  agencies  as  weather,  running 
water  and  frost  and  are  slowly  worn  away.  The  region 
around  Chicago  has  been  a  land  area  so  much  of  the 
time  since  the  Devonian  period  that  any  rocks  depos- 
ited during  times  of  submergence  have  completely  dis- 
appeared. 

Since  the  last  records  now  existing  of  the  early 
geological  history  of  Chicago  were  deposited  on  the 
floor  of  the  Devonian  sea  at  Elmhurst,  so  much  time 
has  passed  that,  if  it  were  computed  in  years,  the  fig- 

[100] 


LEAFLET  7. 


MAP  OF  NORTH  AMERICA  IN  MID-ORDOVICIAN  TIME. 
AFTER  QRABAU. 


Early  Geological  History  op  Chicago  29 

ures  would  be  too  stupendous  for  human  beings  to  real- 
ize their  meaning.  It  has  been  said  that  as  the  astron- 
omer finds  the  mile  too  short  a  measure  for  his 
distance  and  uses  a  larger  unit,  the  light-year,  so  the 
geologist  finds  the  year  too  small  for  his  computation 
of  time  and  uses  the  larger  unit,  a  million  years,  as  a 
basis  of  his  calculations.  The  time  since  the  Devonian 
period  is  a  matter  of  hundreds  of  millions  of  years. 
During  that  time  much  of  the  world's  life  history  has 
arisen  and  passed  on.  The  coal  forests  have  grown 
and  the  coal  beds  have  formed.  Reptiles  have  increased 
from  insignificant  beginnings  until  they  dominated  the 
life  of  the  planet  and  then  decreased  in  importance  to 
their  present  minor  position.  Mammal  and  bird  life 
have  had  their  beginning  and  have  developed  to  their 
present  important  position  in  the  life  of  the  world  and 
there  have  been  other  happenings  of  major  importance. 
The  region  around  Chicago  saw  its  share  of  all  these 
events  but  the  records  are  gone  and  we  can  have  no 
knowledge  of  them  derived  from  local  sources.  We  do 
know,  however,  that  the  progress  of  events  remained 
peaceful  and  uneventful,  for  any  violent  action  of 
geological  agencies  would  surely  have  left  unmistak- 
able evidences  of  its  presence. 

Henry  W.  Nichols 


[101] 


80  Field  Museum  op  Natural  History 


For  those  who  desire  further  information  on  the 
geology  of  this  region,  and  yet  do  not  intend  an  ex- 
haustive study  of  the  subject,  the  following  list  of  ref- 
erences is  provided: 

Pirrson  and  Schuchert. — Text  Book  of  Geology,  Vol.  II. 

Grabau,  Amadeus. — Text  Book  of  Geology,  Vol.  II. 

Chamberlain  and  Salisbury. — Geology,  Vol.  II. 

Geologic  Atlas  op  the  United  States. — Chicago  Folio  No.  81, 
Published  by  the  U.  S.  Geological  Survey.  This  contains  a 
bibliography. 

Weller,  Stuart. — The  Paleontology  of  the  Niagara  Limestone 
in  the  Chicago  area.  Bull.  No.  4  of  the  Natural  History 
Survey,  Chicago  Academy  of  Sciences,  Part  1,  the  Crinoidea 
and  Part  2,  the  Trilobita. 

Weller,  Stuart. — A  peculiar  Devonian  Deposit  in  Northeast- 
ern Illinois.    Journal  of  Geology,  Vol.  VII,  1899. 


[102] 


LEAFLET  7. 


MAP  OF  NORTH  AMERICA  IN  MID-SILURIAN  (NIAGARA)  TIME. 
AFTER  GRABAU. 


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